Output in a format compatible with github annotations so that running in GitHub workflow will annotate code

[somewhatabstract]

When running in a workflow on GitHub Actions, it will be really useful to see inline annotations of code where sync errors occur or fixes are applied (if autofix is run).

See https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/workflow-commands-for-github-actions

I think using this format for output when on GitHub should work 🤞🏻:

"::error file=app.js,line=1::Missing semicolon"

[somewhatabstract]

So, I looked into problem matchers and I don't think we need to modify checksync to get this working.

See https://github.com/actions/toolkit/blob/main/docs/problem-matchers.md and https://github.com/actions/toolkit/blob/main/docs/commands.md#problem-matchers

Create a problem matcher json like this:

{
    "problemMatcher": [
        {
            "owner": "checksync",
            "pattern": [
                {
                    "regexp": "^\\s*(Mismatch|Error|Warning|Migrate|Fix)\\s+([^\\s][^:]*):(\\d+)\\s*$",
                    "file": 2,
                    "line": 3
                },
                {
                    "regexp": "^(.*)$",
                    "message": 1
                }
            ]
        }
    ]
}

Then use that in the workflow that invokes checksync. The path to the matcher is relative to the repo root. So in this example, it's saved in .github as checksync-matcher.json.

echo "::add-matcher::.github/checksync-matcher.json"

[somewhatabstract]

Hmm, it seems we really need the severity to be "warning" or "error" for the problem matcher approach to work. I was hoping that not providing a severity would default to "error" but that isn't happening in my experiments with this.

[somewhatabstract]

It might be the color output confusing things too. Not sure yet. #2118 will allow us to experiment with that.